1. Field of the Invention
This invention relates to a process for producing a conjugated diene monoester by reacting a conjugated diene, a carboxylic acid and oxygen by use of a novel catalyst.
2. Description of the Prior Art
Hitherto, a variety of processes have been proposed for producing unsaturated diesters by reacting a conjugated diene, a carboxylic acid and oxygen in the gas phase in the presence of a catalyst comprising palladium as main component. However, no satisfactory process has ever been reported concerning the process for selectively producing a monoester of conjugated diene as the main product. That is, as examples of the known processes for producing 1-acetoxy-1,3-butadiene, the process in which crotonaldehyde and acetic anhydride are the starting materials (Can. J. of Chem., 38, 1070 (1960)) and the process in which butadiene, acetic acid and oxygen are reacted in the presence of a solid catalyst comprising palladium and potassium acetate as active components (Japanese Patent Publication No. 48,927/74) have been proposed. However, the former process has a fault that the starting materials are difficult to obtain on an industrial scale, and the latter process has a fault that the main product is 1,4-diacetoxybutene-2 and 1-acetoxy-1,3-butadiene is obtained only as a by-product, so that its selectivity is low and its yield is quite low.
The present inventors previously discovered and disclosed in Japanese Patent Application Kokai (Laid-Open) No. 90,210/78 a catalyst comprising as active components (a) palladium, (b) at least one metal selected from the group consisting of aluminum, niobium, tantalum and zirconium, (c) at least one alkali metal halide, and (d) at least one alkali metal carboxylate, as a catalyst for producing a conjugated diene monoester with a high selectivity in a high yield, over a long period of time. The present inventors have further conducted extensive research thereon to find that a conjugated diene monoester can be obtained with a high selectivity in a high yield by using palladium in combination with at least one member selected from the group consisting of magnesium, calcium, barium, cerium, lanthanum, tungsten, copper, zinc, cadmium, boron, thallium, molybdenum, tin, lead, phosphorus, arsenic and selenium and at least one alkali metal carboxylate, as co-catalyst components. It has also been found that combustion reaction can be suppressed and, in addition, a stable catalytic activity can be obtained over a long period of time by further adding an alkali metal halide to the above-mentioned catalyst.